This invention pertains to a novel and unique combination of pyrometric temperature control apparatus including a silicon controlled rectifier power controller incorporating current or power limiting means and other advantageous features. The combination is particularly adapted to the control of temperature in process heating equipment but is not to be construed as limited thereto.
In recent years, static power controllers have been developed which provide what is termed Stepless Control or Phase-Angle Firing Control. These static power controllers usually combine silicon controlled rectifiers and, sometimes diodes, with gating or triggering circuitry which causes the SCR's to conduct during some portion of 180 electrical degrees or half of the sine wave of conventional alternating current electrical power. The control may be either half-wave or full-wave depending upon the configuration of the components of the apparatus. The SCR's are said to "fire" at some angle of the half-sine-wave (firing angle) which may be delayed after the beginning of the sine wave at 0.degree. (electrical) and 0 voltage. The SCR's turn off when the voltage reaches zero at 180.degree. (electrical). The negative half of the voltage sine wave is similarly handled by another SCR which is inversely paralleled with the SCR. In 3-phase configurations, the returning current may be conducted by diodes inversely paralleled with respect to the SCR's.
As the firing angle of the SCR's is increasingly delayed, the portion of a half cycle during which the SCR's conduct becomes shortened. During the blocking portion wherein the SCR does not conduct, no power is delivered. Therefore, as the firing angle is delayed (phased back) less power is transmitted into the load. In prior systems the phase-back angle has commonly been made proportional to the error of temperature of the process in relation to the temperature set-point.
Another type of prior solid state control of electrical power is commonly called "Time Proportional" or "Synchronous Firing" wherein bursts of power comprising one or more complete cycles of alternating current power are permitted to pass through the SCR's and wherein other cycles of the alternating current power are caused to be blocked by the SCR's. A variation in the proportion of cycles fired to the cycles blocked effectively determines the average value of power which is permitted to enter the load. The ratio of fired cycles to blocked cycles is proportional to the deviation of the process temperature from setpoint temperature. Turn on occurs at substantially zero voltage and turn off is likewise at zero voltage.
One important application of phase-angle SCR control is to limit current or power to effectively compensate for such factors as variation in line voltage or load resistance. Without the limits available only with phase-angle SCR control, power into a resistive load would increase as the square of the supply voltage. Also, without the inherent limits applicable with phase-angle control, electrical current would increase inversely as resistance of the load. Thus, phase control is preferred for these applications over Synchronous Firing. The current limit available with phase-angle control even permits transformer coupling of the load. The inrush of current to a transformer coupled load would be untenable with the Synchronous Firing technique.
In spite of the seemingly greater advantages of phase-angle control versus synchronous control, there are serious disadvantages associated with prior adaptations of phase-angle control to process heating and other applications.
It is an object of this invention to minimize or eliminate the disadvantages of phase-angle SCR control of electrical power.
It is another object of the invention to utilize phase-angle control in combination with proportional control to avoid severe phase-angle reductions.
It is another object of the invention to provide a system which employs phase-angle control but which avoids generating radio frequency interference due to large phase-back angles.
It is a further object of the invention to provide a current limited phase-angle control system.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.